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Nitrogen Oxide Atom-Transfer Redox Chemistry; Mechanism of NO(g) to Nitrite Conversion Utilizing ?-oxo Heme-Fe(III)-O-Cu(II)(L) Constructs.

ABSTRACT: While nitric oxide (NO, nitrogen monoxide) is a critically important signaling agent, its cellular concentrations must be tightly controlled, generally through its oxidative conversion to nitrite (NO2(-)) where it is held in reserve to be reconverted as needed. In part, this reaction is mediated by the binuclear heme a3/CuB active site of cytochrome c oxidase. In this report, the oxidation of NO(g) to nitrite is shown to occur efficiently in new synthetic ?-oxo heme-Fe(III)-O-Cu(II)(L) constructs (L being a tridentate or tetradentate pyridyl/alkylamino ligand), and spectroscopic and kinetic investigations provide detailed mechanistic insights. Two new X-ray structures of ?-oxo complexes have been determined and compared to literature analogs. All ?-oxo complexes react with 2 mol equiv NO(g) to give 1:1 mixtures of discrete [(L)Cu(II)(NO2(-))](+) plus ferrous heme-nitrosyl compounds; when the first NO(g) equiv reduces the heme center and itself is oxidized to nitrite, the second equiv of NO(g) traps the ferrous heme thus formed. For one ?-oxo heme-Fe(III)-O-Cu(II)(L) compound, the reaction with NO(g) reveals an intermediate species ("intermediate"), formally a bis-NO adduct, [(NO)(porphyrinate)Fe(II)-(NO2(-))-Cu(II)(L)](+) (?max = 433 nm), confirmed by cryo-spray ionization mass spectrometry and EPR spectroscopy, along with the observation that cooling a 1:1 mixture of [(L)Cu(II)(NO2(-))](+) and heme-Fe(II)(NO) to -125 °C leads to association and generation of the key 433 nm UV-vis feature. Kinetic-thermodynamic parameters obtained from low-temperature stopped-flow measurements are in excellent agreement with DFT calculations carried out which describe the sequential addition of NO(g) to the ?-oxo complex.

SUBMITTER: Hematian S

PROVIDER: S-EPMC4469181 | biostudies-literature | 2015 May

REPOSITORIES: biostudies-literature

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Nitrogen Oxide Atom-Transfer Redox Chemistry; Mechanism of NO(g) to Nitrite Conversion Utilizing μ-oxo Heme-Fe(III)-O-Cu(II)(L) Constructs.

Hematian Shabnam S Kenkel Isabell I Shubina Tatyana E TE Dürr Maximilian M Liu Jeffrey J JJ Siegler Maxime A MA Ivanovic-Burmazovic Ivana I Karlin Kenneth D KD

Journal of the American Chemical Society 20150514 20

While nitric oxide (NO, nitrogen monoxide) is a critically important signaling agent, its cellular concentrations must be tightly controlled, generally through its oxidative conversion to nitrite (NO2(-)) where it is held in reserve to be reconverted as needed. In part, this reaction is mediated by the binuclear heme a3/CuB active site of cytochrome c oxidase. In this report, the oxidation of NO(g) to nitrite is shown to occur efficiently in new synthetic μ-oxo heme-Fe(III)-O-Cu(II)(L) construct ...[more]

PMID: 25974136

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Project description:Current interest in copper/dioxygen reactivity includes the influence of thioether sulfur ligation, as it concerns the formation, structures, and properties of derived copper-dioxygen complexes. Here, we report on the chemistry of {L-Cu(I)}2-(O2) species L = (DMM)ESE, (DMM)ESP, and (DMM)ESDP, which are N3S(thioether)-based ligands varied in the nature of a substituent on the S atom, along with a related N3O(ether) (EOE) ligand. Cu(I) and Cu(II) complexes have been synthesized and crystallographically characterized. Copper(I) complexes are dimeric in the solid state, [{L-Cu(I)}2](B(C6F5)4)2, however are shown by diffusion-ordered NMR spectroscopy to be mononuclear in solution. Copper(II) complexes with a general formulation [L-Cu(II)(X)](n+) {X = ClO4(-), n = 1, or X = H2O, n = 2} exhibit distorted square pyramidal coordination geometries and progressively weaker axial thioether ligation across the series. Oxygenation (-130 °C) of {((DMM)ESE)Cu(I)}(+) results in the formation of a trans-μ-1,2-peroxodicopper(II) species [{((DMM)ESE)Cu(II)}2(μ-1,2-O2(2-))](2+) (1(P)). Weakening the Cu-S bond via a change to the thioether donor found in (DMM)ESP leads to the initial formation of [{((DMM)ESP)Cu(II)}2(μ-1,2-O2(2-))](2+) (2(P)) that subsequently isomerizes to a bis-μ-oxodicopper(III) complex, [{((DMM)ESP)Cu(III)}2(μ-O(2-))2](2+) (2(O)), with 2(P) and 2(O) in equilibrium (K(eq) = [2(O)]/[2(P)] = 2.6 at -130 °C). Formulations for these Cu/O2 adducts were confirmed by resonance Raman (rR) spectroscopy. This solution mixture is sensitive to the addition of methylsulfonate, which shifts the equilibrium toward the bis-μ-oxo isomer. Further weakening of the Cu-S bond in (DMM)ESDP or substitution with an ether donor in (DMM)EOE leads to only a bis-μ-oxo species (3(O) and 4(O), respectively). Reactivity studies indicate that the bis-μ-oxodicopper(III) species (2(O), 3(O)) and not the trans-peroxo isomers (1(P) and 2(P)) are responsible for the observed ligand sulfoxidation. Our findings concerning the existence of the 2(P)/2(O) equilibrium contrast with previously established ligand-Cu(I)/O2 reactivity and possible implications are discussed.

Dataset Information

Nitrogen Oxide Atom-Transfer Redox Chemistry; Mechanism of NO(g) to Nitrite Conversion Utilizing ?-oxo Heme-Fe(III)-O-Cu(II)(L) Constructs.

Publications

Nitrogen Oxide Atom-Transfer Redox Chemistry; Mechanism of NO(g) to Nitrite Conversion Utilizing μ-oxo Heme-Fe(III)-O-Cu(II)(L) Constructs.

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